South Korea formally accused North Korea on Thursday of responsibility for the sinking of a South Korean warship in March, killing 46 sailors, in one of the deadliest provocations since the two countries ended the Korean War in a truce in 1953.

In an uncharacteristicallyswift reaction, North Korea called the South Korean conclusion a “fabrication” and threatened to respond to sanctions with “strong measures, including a full-scale war.” It also offered to send a team of “inspectors” to South Korea to ch allege its investigation.

South Korea saw “no other plausible explanation” than a North Korean attack. “The evidence points overwhelmingly to the conclusion that the torpedo was fired by a North Korean submarine,” a team of military and civilian investigators from South Korea, the United States, Britain, Australia and Sweden said in a statement. “There is no other plausible explanation.”

The widely expected conclusion was bound to escalate tensions between the Koreas and to intensify an international debate on how to punish the North.

North Korea, which was also found responsible in 1987 for the bombing of a South Korean commercial jet that killed 115 people, has denied involvement in the sinking on March 26 of the 1,200-ton ship, the Cheonan, which occurred near the countries’ disputed western sea border. North Korea has vowed to retaliate against any attempts to link it to the sinking; it recently threatened to shut down the border, raising the possibility of trapping 1,000 South Korean workers at a joint industrial park in the North Korean city of Kaesong.

The South insists that the sinking of its warship deserves punishment — through unilateral and United Nations economic sanctions — because it violates the United Nations charter and the 1953 Korean War armistice.